part 01 - fnirs: a cost-effective and robust technique for measuring neuroactivation 1
TRANSCRIPT
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Part 01 - fNIRS: a cost-effective and robust technique for measuring neuroactivation
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NIRx Offices (Year Established)
New York, USA (1999)
Berlin, Germany (2006)
Los Angeles, USA (2013)
São Paulo, Brazil (2014)
Istanbul, Turkey (Pending, October 2014)
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What is NIRS?
NIRS = Near- InfraRed Spectroscopy
A form of Optical Imaging.
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… can be used to identify molecular structures.
The interaction of EM radiation with matter…
A bit about Spectroscopy
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The oxygen transporter in the blood of vertebrate animals.
The molecule of interest: Hemoglobin (Hb)
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Neural activation↓
Metabolic demand↓
Increased blood flow↓
Increase in oxy-hemoglobin&
Wash-out of deoxy-hemoglobin
The Hemodynamic Response
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To discriminate between HbO and HbR, NIRS systems use multiple wavelengths in measurements. (example: 760nm & 850nm)
EM absorption of oxy-Hb and deoxy-Hb
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Best frequencies for physiologic chromophores are partially within the red & NIR spectrum
Lower absorption of light = Better transmission through tissue and bone
Well suited for modern LED's or lasers (650…900nm)
Optimal Light Frequencies of NIRS
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Skin and bone are mostly transparent to near-infrared light and do not significantly impede the signal.
Practical Example
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Relies on properties of absorption and scattering of photons.
Principles of Optical Imaging
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The sensitivity profile light conforms to an elliptical “banana” shape as it travels from source to detector. This is known as a photon banana.
Principles of Optical Imaging
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A single light source sends light out indiscriminately, with signal detected at potentially infinite positions.
Principles of Optical Imaging
Hemoglobin Hemoglobin
DetectorDetector
Detector Detector
Light Source
Ambient Light Ambient Light
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Source
Detector
Collects functional information from top 1cm of cortex, using “nearest neighbor” source-detector measurements.
One data channel is produced per wavelength used per source/detector pairing.
NIRS Topography
The source-to-detector pairing distance effects the depth of the photon banana penetration.
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S
D
S
D
S
D
S
D
S
2 – 3 cm‘Nearest-neighbor’ measurements are done with spacing of 2-3cm between sources and detectors.
S
D
= light source
= light detector
= data channel
NIRS Topography
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Source
Detector 1
Detector 2
Eliminates systemic interference of superficial origin (closest channel => mostly non-cortical signal)
NIRS Tomography
Like Topography, collects functional information from top 1cm of cortex, but uses multi-distance measurements with a single source sending light to multiple detectors in 1-dimention.
Multi-distance measurements allow for depth discrimination of signal
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Kohl M ,.. , Dirnagl U. Phys. Med. Bio. 2000
oxy-Hb (HbO)
deoxy-Hb (Hb)
x10-3
time in [s]
0 2 4 6 8 10 12 14 16
-1
0
1
2
D c
once
ntra
tion
[mM
]
Stimulation period
The NIRS Hemodynamic Response
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• Spatial resolution of neuro-activation within the mm-to-cm range (depending on probe density)
• Excellent sensitivity to Hb and HbO
• Excellent sampling rate (several Hz to 60Hz+)
• Measures top 1cm of cortex
• Greatest light transmission is on forehead (prefrontal) due to lack of hair, supporting vascular structure, and sinuses
Physical and Biological Principles
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How does NIRS compare to more well-known functional brain-imaging modalities?
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BOLD fMRI
• Identifies deoxyhemoglobin as well as detailed tissue structure from entire brain
• Hundreds of thousands of peer-reviewed publications
• Large form-factor, noisy operation, very expensive systems and operation
• Confined environment
Functional Hemodynamic Modalities
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fNIRS• Identifies deoxyhemoglobin as well as oxyhemoglobin
(total hemoglobin derived) from top 1cm of cortex (deeper at forehead)
• Thousands of peer-reviewed publications, but rapidly increasing in number
• Relatively small form-factor (portable options available)
• Silent system operation (ideal for sensitive subjects)
• System cost is a fraction of the cost of fMRI, operational cost is very low
Functional Hemodynamic Modalities
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Comparison - fMRI vs. fNIRS
• Spatial mapping of activation area in response to right-hand activity
NIRS,D HbR fMRI
BOLD
Jan Mehnert, OHBM Annual Meeting, 2010
LL
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fNIRS fMRI
seconds
Jan Mehnert, OHBM Annual Meeting, 2010
Most Significant Activation Response
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EEG
• Measures cortical electrophysiology from scalp
• Tens of thousands of peer-reviewed publications
• Excellent temporal resolution (sampling rates 2kHz+)– Measures very quick paradigms related to action potentials (P300,
N200, etc.) as well as frequency bands (alpha, beta, etc.)
• Relatively poor spatial resolution (several cm)
• Portable/wearable systems available, but highly sensitive to motion artifacts
Small Form-Factor Neuro-Modalities
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Small Form-Factor Neuro-Modalities
fNIRS
• Measures hemodynamic cortical activity - Similar headgear and form-factor to EEG
• Lower temporal resolution than EEG (fNIRS typically ~5-10Hz)– Though lower, the sampling rate for fNIRS is more than sufficient for
measuring hemodynamic responses of interest (.01 - .2Hz)
• Greater spatial resolution than EEG (fNIRS is ~6mm)
• Better noise performance than EEG with even very extreme movements and muscle usage
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NIRS technology offers:• Compact, low cost sensing hardware• Noninvasive, continuous measures• Highly configurable and scalable platforms• Information-rich sensing data
– BOLD-like response without the magnet– Source localization >> EEG– Minimal sensitivity to movement artifacts
Summary of NIRS
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Thanks!
... Questions?